Bone Density and Space Travel

Gravity it keeps you on the ground, and keeps your bones healthy

In 1892, Dr. Julius Wolff reported the changes that are made in bone based on loading that is placed on the body. Most specifically, by placing the highest tolerable loads on the skeletal structure in an axial format. This means load placed on the long bones of the body, from one end to the other, not from the sides. Nominal loading can maintain bone mass for short periods (weeks to months) of time with little loss of bone mass density, but longer can allow the natural shedding of bone to continue to happen, with the newer bone intended to replace the old bone not growing because it doesn’t have the stimulation to build itself.

This is most apparent in two different populations, each of whom represent the extremes of bone mass loss:

Individuals who are so compromised, they cannot move on their own (confined to bed rest)

Astronauts

CAPE CANAVERAL, FL - OCTOBER 15: Space Shuttle Discovery astronauts, (L-R) Commander Steve Lindsey, pilot Eric Boe, and mission specialists Al Drew, Mike Barratt, Tim Kopra, and Nicole Stott, walk out of the Operations and Checkout Building and load into the astronaut van before being transported to the launch pad October 15, 2010, in Cape Canaveral, Florida. The astronauts were participating in the terminal countdown demonstration test in preparation for their scheduled November launch to the International Space Station. (Image credit: Getty Images via @daylife)

Astronauts can be an easier example to look at, as typically they are without the other adverse health conditions that the bed rest group is with, so the number of compounding factors are significantly less. The astronauts that have spent extended time in a zero gravity environment have rapid bone mass density loss, and are often not able to walk after a stay in space. Not being able to place loading on the body effects many other systems, for example, loss of muscle mass and loss of strength can occur after just 2 weeks in bed. In 6 weeks, the heart muscle, or cardiac mass, is decreased, and in 12 weeks, bone mineral density may show a decrease (Leblanc, et al. 1990).

Research on both astronauts and the bed rest populations has helped scientists better understand and refine the understanding of bone mass function, and how both losses and gains of bone mass density can be controlled.